Systems and Methods Implementing Layers of Devitrified Metallic Glass-Based Materials

ABSTRACT

Systems and methods in accordance with embodiments of the invention implement layers of devitrified metallic glass-based materials. In one embodiment, a method of fabricating a layer of devitrified metallic glass includes: applying a coating layer of liquid phase metallic glass to an object, the coating layer being applied in a sufficient quantity such that the surface tension of the liquid phase metallic glass causes the coating layer to have a smooth surface; where the metallic glass has a critical cooling rate less than 10 6  K/s; and cooling the coating layer of liquid phase metallic glass to form a layer of solid phase devitrified metallic glass.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application claims priority to U.S. Provisional Application No. 62/293,210, filed Feb. 9, 2016, the disclosure of which is incorporated herein by reference.

STATEMENT OF FEDERAL FUNDING

The invention described herein was made in the performance of work under a NASA contract NNN12AA01C, and is subject to the provisions of Public Law 96-517 (35 U.S.C. 202) in which the Contractor has elected to retain title.

FIELD OF THE INVENTION

The present invention generally regards layers of devitrified metallic glass-based materials, and techniques for fabricating such layers.

BACKGROUND

Metallic glasses, also known as amorphous metals, have generated much interest for their potential as robust engineering materials. Metallic glasses are characterized by their disordered atomic-scale structure in spite of their metallic constituent elements—i.e. whereas conventional metallic materials typically possess a highly ordered atomic structure, metallic glasses are characterized by their disordered atomic structure. Notably, metallic glasses typically possess a number of useful material properties that can allow them to be implemented as highly effective engineering materials. For example, metallic glasses are generally much harder than conventional metals, and are generally tougher than ceramic materials. They are also relatively corrosion resistant, and, unlike conventional glass, they can have good electrical conductivity.

Nonetheless, the manufacture and implementation of metallic glasses present challenges that limit their viability as engineering materials. In particular, metallic glasses are typically formed by raising a metallic glass above its melting temperature, and rapidly cooling the melt to solidify it in a way such that its crystallization is avoided, thereby forming the metallic glass. The first metallic glasses required extraordinary cooling rates, e.g. on the order of 10⁶ K/s, to avoid crystallization, and were thereby limited in the thickness with which they could be formed because thicker parts could not be cooled as quickly. Indeed, because of this limitation in thickness, metallic glasses were initially largely limited to applications that involved coatings. Accordingly, the present state of the art can benefit from improved techniques for implementing layers of metallic glass.

SUMMARY OF THE INVENTION

Systems and methods in accordance with embodiments of the invention implement layers of devitrified metallic glass-based materials. In some embodiments, a method of fabricating a layer of a devitrified metallic glass includes:

-   -   providing a liquid phase metallic glass having a critical         cooling rate of less than 10⁶ K/s;     -   applying a liquid phase metallic glass to an object, wherein         applying the coating layer comprises immersing at least a         portion of the object such that the object is wetted by the         liquid phase metallic glass to form a layer of liquid phase         metallic glass on the outer surface thereof; and     -   solidifying the layer of liquid phase metallic glass-forming         alloy such that a solid phase devitrified metallic glass-forming         coating is formed therefrom.

In other embodiments the grain size of the coating is nanocrystalline with an average grain size from 10 nanometers to 1000 nanometers.

In still other embodiments the grain size is greater than 1 micrometer.

In yet other embodiments the coating crystallizes during cooling from the liquid phase.

In still yet other embodiments the method further includes applying an external heat source to heat the object during solidifying.

In still yet other embodiments the method further includes

-   -   quenching the liquid phase at a cooling rate faster than the         critical cooling rate of the liquid phase metallic glass to form         a solid phase metallic glass coating;     -   heating the solid phase metallic glass coating to a processing         temperature above the glass transition temperature of the         metallic glass and holding the metallic glass coating at the         processing temperature to form a devitrified metallic         glass-forming coating; and     -   cooling the devitrified metallic glass-forming coating to below         the glass transition temperature.

In still yet other embodiments the devitrified coating has a hardness that is at least 10% higher than the amorphous phase of the same alloy.

In still yet other embodiments the devitrified coating has a Young's modulus that is at least 10% higher than the amorphous phase of the same alloy.

In still yet other embodiments the metallic glass-forming alloy is applied to an object that is at a higher temperature than the liquidus temperature of the metallic glass-form ing alloy causing it to melt and wet the object.

In still yet other embodiments the devitrified coating has a lower surface roughness than the object to which the liquid phase metallic glass is applied.

In still yet other embodiments the immersion of the object comprises one of the methods selected from the group consisting of dipping, pouring and spraying.

In still yet other embodiments the object being coated is made from metal, polymer, ceramic, glass, or mixtures thereof.

In still yet other embodiments the thickness of the coating layer is greater than 50 micrometers.

In still yet other embodiments the thickness of the coating layer is greater than 1 mm.

In still yet other embodiments the coating process is done under a vacuum or inert environment.

In still yet other embodiments the coating does not exhibit a glass transition temperature when heated.

In still yet other embodiments the method further includes spinning the object during the applying and solidifying.

In still yet other embodiments the object comprises one of aluminum, titanium, steel, cobalt, graphite, quartz, silicon carbide, and mixtures thereof.

In still yet other embodiments the metallic glass has a melting temperature of less than 800° C.

Some other embodiments are also directed to methods of fabricating a layer of devitrified metallic glass including:

-   -   providing a liquid phase metallic glass having a critical         cooling rate of less than 10⁶ K/s and a melting temperature of         less than 800° C.;     -   applying a liquid phase metallic glass to an object, wherein         applying the coating layer comprises immersing at least a         portion of the object such that the object is wetted by the         liquid phase metallic glass to form a layer of liquid phase         metallic glass on the outer surface thereof; and     -   solidifying the layer of liquid phase metallic glass-forming         alloy such that a solid phase devitrified metallic glass-forming         coating is formed therefrom.

Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention. A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be more fully understood with reference to the following figures and data graphs, which are presented as exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention, wherein:

FIG. 1 illustrates an exemplary TTT curve for a metallic glass-based material.

FIG. 2A illustrates a process for forming a layer of metallic glass in accordance with embodiments of the invention.

FIGS. 2B and 2C illustrate exemplary TTT curves for: (2B) moderate, and (2C) poor glass forming metallic glass-based materials, in accordance with embodiments of the invention.

FIG. 3A illustrates a process for forming a layer of metallic glass in accordance with embodiments of the invention.

FIG. 3B illustrates an exemplary TTT curve for a good glass forming metallic glass-based material, in accordance with embodiments of the invention.

FIGS. 4A and 4B illustrate how a coating layer of metallic glass can be developed to mask a rough object surface in accordance with embodiments of the invention.

FIG. 5 illustrates dipping an object in a bath of liquid phase metallic glass to develop a layer of metallic glass on the object in accordance with embodiments of the invention

FIGS. 6A to 6C illustrate spinning an object having a coating layer of liquid phase metallic glass to facilitate the wetting of the object and to eliminate excess liquid in accordance with embodiments of the invention.

FIG. 7 illustrates dipping an object in a bath of liquid phase metallic glass within and inert atmosphere to develop a layer of metallic glass on the object in accordance with embodiments of the invention

FIG. 8 illustrates pouring liquid phase metallic glass over an object to develop a layer of metallic glass on the object in accordance with embodiments of the invention.

FIG. 9 illustrates coating a cell phone casing with a layer of metallic glass in accordance with embodiments of the invention.

FIG. 10 illustrates spraying the inside of a piping with a layer of liquid phase metallic glass in accordance with embodiments of the invention.

FIGS. 11A and 11B illustrate fabricating a layer of metallic glass by pouring liquid phase metallic glass over a substrate, cooling the liquid phase metallic glass, and separating the solidified metallic glass from the substrate.

FIG. 12 illustrates using a rolling wheel to help form a liquid phase layer of metallic glass that has been poured on a substrate in accordance with embodiments of the invention.

FIG. 13 provides the Ti—Be binary phase diagram showing a very deep eutectic temperature at 30 atomic % Be.

FIG. 14 provides DSC curves showing traces from a BMG and three BMG composites, each having a liquidus temperature of below ˜770° C., in accordance with embodiments of the invention.

FIG. 15 provides Xray data plots showing: (Left) a fully amorphous x-ray scan showing no crystalline Bragg peaks, indicating a glass, (Center) the same alloy cooled slightly slower with at least one crystalline peak, indicating that the alloy is partially crystalline, and (Right) the alloy has cooled so slowly that it is now fully crystalline in accordance with embodiments of the invention.

FIG. 16 provides a table of properties of exemplary amorphous (A), crystalline (X), and composite (C) metallic glass-based materials in accordance with embodiments of the invention.

FIG. 17 provides data from a hardness measurement for an exemplary metallic-glass based material in glass and crystalline form, in accordance with embodiments, in comparison with a Ti-6-4 material.

DETAILED DESCRIPTION

Turning now to the drawings, systems and methods for implementing layers of devitrified metallic glass-based materials are illustrated. In many embodiments the systems and methods comprise dipping an object into a molten reservoir of a metallic glass-based material to form a layer on a portion of the object, and cooling the layer of metallic glass-based material at a rate slow enough to ensure devitrification of the material.

For the purpose of this invention, amorphous metal is a multi-component metal alloy that exhibits an amorphous (non-crystalline) atomic structure. These alloys can also be called metallic glasses, as they exhibit a glass transition temperature. A de-vitrified metallic glass is one that has a fully or mostly crystalline structure either due to insufficient cooling from the liquid or from heating with the intent of crystallization. For the purposes of this patent application, the term ‘metallic glass-based material’ shall be interpreted to be inclusive of ‘amorphous alloys’, ‘metallic glasses’, and ‘metallic glass composites’, except where otherwise noted. Metallic glass composites are characterized in that they possess the amorphous structure of metallic glasses, but they may also include crystalline phases of material within the matrix of the amorphous structure.

Amorphous metals are a unique class of metal alloys known for having remarkable mechanical properties due to their lack of microstructure (e.g., an ‘amorphous’ microstructure). However, to attain this ‘amorphous’ microstructure it is necessary to rapidly cool the alloy from its molten state to below its solidification temperature at a rate known as the ‘critical cooling rate’ to avoid crystallization of the material. A diagram of a typical crystallization curve for a metallic glass material is shown in FIG. 1. A ‘critical cooling rate’ refers to how fast a liquid phase metallic glass must be cooled in order to avoid the crystallization region shown in FIG. 1 and form the corresponding solid phase metallic glass, i.e., having an amorphous crystalline structure. Typical cooling rates for metallic glasses range from above 10⁶ K/s, and even low critical cooling rates are typically above 100 K/s.

The critical cooling rate of a metallic glass is associated with a material's ‘glass forming ability,’ a term that references a measure as to how easy it is to form a solid phase metallic glass. In typical applications, it is desirable to use metallic glasses having low critical cooling rates because such materials provide the user more time to cool the material during processing before it crystallizes. This additional time can be used for forming, or to obtain thicker coatings or objects having larger volumes. However, as even the best glass forming alloys have critical cooling rates for metallic-glass materials that are on the order of 1-10 K/s, having to operate within this narrow region greatly limits the widespread commercialization of these materials, and how these materials may be integrated into applications where they otherwise might offer a substantial benefit compared with existing materials (either in cost, mechanical properties or processing ability).

For example, amorphous metals have been cast into net-shaped parts, similar to plastics, that can be used for electronic cases, golf clubs, medical devices etc. These alloys are typically referred to as bulk metallic glasses (BMGs). However, even these ‘bulk’ alloys are limited to cast parts having thicknesses on the order of millimeters or centimeters. Most amorphous metals are limited to use in thin sheets with thicknesses between 10-100 micrometers produced through melt spinning liquid onto a rotating copper wheel. These ribbons have excellent magnetic properties and have been used as transformer cores and as anti-theft identification tags, but the materials cannot be used to make thicker objects.

Another area where amorphous metals have experienced large growth is in coatings. Many techniques may be used to implement layers of metallic glass, e.g. metallic glass coatings on objects. To account for the critical cooling rates of most metallic glasses, these coatings are typically produced through a thermal spraying process, such as high velocity oxy-fuel (HVOF) or wire arc spraying, and they produce a hard and durable coating primarily used for protecting pipes and drill bits in the oil and gas industry. Other techniques for fabricating coatings from amorphous metals have also been attempted, including plating, evaporation and sputtering. However, many of the techniques that have been used thus far exhibit a number of shortcomings. For example, thermal spraying techniques have been used to implement metallic glass coatings. Thermal spraying techniques generally regard spraying heated material onto an object to establish a coating. In some thermal spraying techniques, metallic glass in a powdered form of micrometer sized particles is sprayed onto the object to be coated. In other thermal spraying techniques, metallic glass in a wire form is heated to a molten state and thereby applied to the object to be coated. However, these thermal spraying techniques are limited insofar as they usually result in a coating that has a very rough surface finish; in many instances it is desirable for the coating to have a smooth finish. Moreover, thermal spraying techniques generally can be fairly time-consuming. Additionally, these techniques may be fairly expensive to implement because the feedstock, e.g. the metallic glass in powdered form, can be costly. Sputtering techniques and chemical vapor deposition techniques have also been used to implement metallic glass coatings; but these techniques have their own shortcomings. For example, sputtering techniques and chemical vapor deposition techniques generally regard a layer by layer deposition of material on an atomic scale. With this being the case, such processes can be extremely slow. Moreover, the thickness of the coating layer can be substantially limited, in many cases to less than 10 micrometers.

One technique that has not been explored is immersion coating. Immersion coating involves the dipping or immersion of parts into molten baths of glass-forming alloys to form a coating. This technique offers a solution to an area of coating technology that cannot be easily solved using spray coating, plating or evaporation. In this case, a part is submerged into a bath of molten metal in a vacuum chamber or is sprayed with a liquid of the same metal (different from thermal spray coating which uses atomization). After a short time, the part is removed from the bath or continuously passes through the bath and is cooled via blowing gas, conduction into the part, quenching in a liquid or radiation in air. This technique allows for a coating of between 0.1-1 millimeters to be applied to a part without having to use thermal spraying or deposition techniques, drastically decreasing the coating time. Moreover, because the coating goes on as a liquid layer and is allowed to drip off, the coating is smooth and durable after hardening.

The reason immersion coating has not been considered for use with metallic glass-based materials is that current techniques are evaluated with an extensive focus on ensuring a fast cooling rate to facilitate the formation of the solid phase metallic glass. The instant application identifies that many metallic glass-based materials have properties in their devitrified (i.e., crystallized) form that are themselves novel, and that many metallic glass-based materials have the further advantage of having very low melting temperatures (e.g., <1200° C.), which make these devitrified metallic glass-based materials uniquely suited for use as coating materials. Accordingly, in the current technique, coatings are applied in such a way to ensure that they do not form a fully amorphous layer but rather crystallize into a mostly or fully crystalline coating, which results in properties that are similar, but completely distinct from a fully amorphous metallic-glass based material.

Thus, in many embodiments, a liquid phase metallic glass-based material is applied to an object in a manner that allows the metallic glass-based material to wet the object, and the liquid phase metallic glass is thereafter allowed to cool, but at a rate slow enough such that the metallic glass devitrifies to form a layer of solid phase devitrified metallic glass coating. The layer of solid phase devitrified metallic glass can form in spite of the fact that a relatively substantial volume of liquid phase metallic glass may be used to coat the object, because the normal limitations concerning critical cooling rates do not apply. Processes and materials for fabricating such devitrified metallic glass layers are discussed in greater detail below.

Fabricating Devitrified Metallic Glass Layers

Many embodiments are directed to systems and methods for forming layers and coatings of devitrified metallic glass-based materials. In various such embodiments, liquid phase metallic glass is applied such that it wets an object in relatively substantial volumes by immersion, and is thereafter forced to cool at a cooling rate slower than the material's critical cooling rate such that a solid phase devitrified metallic glass layer is formed. As discussed in the embodiments below, several methods may be used to form the devitrified metallic glass-based material coatings.

As shown in FIGS. 2A to 2C, in various embodiments the immersion technique involves melting a volume of metallic-glass forming alloy into a liquid. Once a molten metallic-glass forming alloy is formed the object is immersed in the molten alloy feedstock such that a volume or quantity of liquid phase metallic glass is deposited on the outer surfaces of the object.

Once a suitable quantity of liquid phase metallic glass is applied to the outer surface of the object the layer of liquid phase metallic glass is then cooled, however, rather than quenching the alloy rapidly as is typically required of processing metallic glasses, in embodiments the temperature of the liquid phase metallic glass layer is cooled sufficiently slowly to ensure the formation of a solid phase devitrified metallic glass layer. This generally requires a cooling rate slower than the critical cooling rate, such that the metallic glass alloy passes through its crystallization region, as shown in FIGS. 2B and 2C. As will be discussed in greater detail below in reference to specific types of metallic glass-based materials, different metallic glass materials will have different glass forming abilities and thus critical cooling rates. FIG. 2B demonstrates a TTT curve from a moderate glass forming alloy where a specific cooling rate barely passes through the nose of crystallization, while the TTT curve in FIG. 2C from a weak glass forming alloy shows that the same cooling rate passes well into the crystallization region of the material. While both will devitrify in the moderate class former, where the crystallization region is only barely entered the crystallization will occur more slowly resulting in a nanocrystalline structure, whereas in the poor former the coating will have a grain size larger than the nanometer scale. Accordingly, by selecting material and/or cooling rate it is possible to control the size and nature of the crystallization domains of the final coating.

Although FIGS. 2A to 2C describe a process where a single cooling step is used, it should be understood that other multi-step processes may be used to obtain solid phase devitrified metallic glass layers. For example, as summarized in FIGS. 3A and 3B, in various embodiments an alloy may be used that is an excellent glass forming alloy where quenching the coating from the liquid to below the glass transition forms an amorphous layer. In such cases the layer must then be heated after quenching to alloy for crystallization of the coating before quenching again back to room temperature.

Accordingly, in such embodiments a process for forming solid phase devitrified metallic glass layers may involve melting a volume of metallic-glass forming alloy into a liquid. Once a molten metallic-glass forming alloy is formed, immersing an object in the molten alloy feedstock such that a volume or quantity of liquid phase metallic glass is deposited on the outer surfaces of the object. Once a suitable quantity of liquid phase metallic glass is applied to the outer surface of the object the layer of liquid phase metallic glass is then cooled to form a solid phase metallic glass layer. This generally requires a cooling rate faster than the critical cooling rate (as shown in FIG. 3B).

Any suitable technique can be used to cool the layer of liquid phase metallic glass. For example, the metallic glass layer can be spun to facilitate cooling by convection. Cooling gases may also be used to cool the liquid phase metallic glass. In some embodiments, the cooling of the liquid phase metallic glass layer occurs largely by thermal conduction, e.g. through object that was coated. Of course, although certain techniques for cooling the liquid phase cooling layer are mentioned, it should of course be understood that any suitable technique(s) for cooling the liquid phase metallic glass layer can be implemented in accordance with embodiments of the invention. In many embodiments, the application of the liquid phase metallic glass and its cooling is done with such rapidity, that even where the object that is coated with liquid phase metallic glass has a lower melting point than the metallic glass, a metallic glass layer can still be developed on the object, i.e. the liquid phase metallic glass does not melt the object. In particular, liquid phase metallic glass can be applied to the object in relatively substantial volumes and cooled all prior to the thermal energy diffusing through the coated object to melt it.

In such a process, as shown in FIG. 3A, once the solidified metallic glass coating is obtained, the object is reheated to above the glass transition temperature such that the material passes through its crystallization region (as shown in FIG. 3B) thus resulting in devitrification of the solid phase metallic glass-based layer. Once devitrification has occurred, the coating is quenched again to form the final coated object.

Regardless of the specific process chosen, in the final step the heating and/or cooling rate of the metallic glass layer is controlled to ensure devitrification of the metallic glass-based material. Any suitable technique can be used to control the cooling of the layer of liquid phase metallic glass to ensure devitrification. For example, the coated object may have its temperature purposefully elevated, such as by an oven, kiln or other heating element. In addition, suring cooling other techniques may be used to improve the quality of the coating finish. For example, the metallic glass layer can be spun to eliminate excess liquid, which can inhibit the quality of the surface finish. The formation of layers of metallic glass can also be highly sensitive to the development of oxide layers or other contamination that can adversely impact the final material properties. In particular, many CuZr-based alloys, Ti-based alloys, and Zr-based alloys are sensitive in this manner. Thus, in many embodiments, the application of liquid phase metallic glass and its cooling may occur in an inert environment. For instance, the application of the liquid layer and its cooling can occur in a chamber that is substantially filled with one of: argon, helium, neon, nitrogen and/or mixtures thereof (argon, helium, neon, and nitrogen being relatively inert elements).

The ability to develop metallic glass layers without reference to critical cooling rates allows for the use of relatively substantial volumes of liquid phase metallic glass. This can offer many advantages. For example, using relatively substantial volumes of liquid phase metallic glass can allow thicker layers of metallic glass to form, which can provide for greater structural integrity. Indeed, where a part is coated in a metallic glass layer, if the metallic glass layer is sufficiently thick, the part with the coated layer can perform in many ways as if it were entirely constituted from the metallic glass.

Additionally, using relatively substantial volumes of liquid phase metallic glass can allow for the final layer of metallic glass to have a smooth finish, which in many instances can be desirable. For example, smooth finishes generally provide for appealing aesthetics. Moreover, smooth surface finishes can also be used to facilitate laminar flow, e.g. where the inside of a pipe that is to facilitate the transportation of liquid has a smooth finish. Furthermore, the smooth layer of metallic glass can be used to mask the rough surface of the object that was coated. FIGS. 4A and 4B illustrate this principle. In particular, FIG. 4A depicts a diagram showing a substrate with a rough surface finish, which is then coated by metallic glass, to develop a smooth surface finish in accordance with embodiments of the invention. In effect, the liquid phase metallic glass, when applied, can fill into any pores or openings that define the substrate's rough surface. FIG. 4B provides a set of images of this result with respect to a machined Ti-6-4 surface. As seen in FIG. 4B the metallic glass coated Ti-6-4 surface appears much more smooth than the original part that was coated in the metallic glass, and particularly eliminates the machining flaws.

Accordingly, in some embodiments, the quantity of liquid phase metallic glass that is applied is such that the surface tension of the liquid phase metallic glass causes the coating layer to have a smooth surface, and in many embodiments, a sufficient quantity of liquid phase metallic glass is applied such that the surface of the developed coating layer is smoother than that of the object that was coated with the coating layer. The surface tension of a liquid refers to its contractive tendency; it is generally caused by the cohesion of similar molecules, and is responsible for many of the behaviors of liquids. Thus, when a sufficient quantity of liquid phase metallic glass is applied, cohesive interactions between the constituent elements can cause an even distribution of the coating layer across the surface of the layer, i.e. the coating layer can have a smooth surface. By contrast, when thermal spraying techniques are used to implement layers of metallic glass, the metallic glass is typically sparsely distributed on to the object to be coated such that surface tension effects do not take place across the coating layer; as a consequence, thermal spraying techniques generally result in rough surface finishes.

Of course, it should be noted that, any suitable measure may be used to ensure the application of a relatively substantial volume of liquid phase metallic glass in accordance with embodiments of the invention. For instance, in some embodiments, a sufficient quantity of liquid phase metallic glass is applied such that a coating layer having a thickness of greater than approximately 50 micrometers develops. For example, in many embodiments liquid phase metallic glass is applied to develop a coating layer having a thickness as high as 1 mm or more. Of course, although a particular threshold quantity is mentioned, it should be understood that any suitable threshold value can be implemented in accordance with embodiments of the invention.

Techniques for applying liquid phase metallic glass are now discussed below.

Fabricating Metallic Glass Layers

Liquid phase metallic glass can be applied by immersion to objects in many ways in accordance with embodiments of the invention. For example, as shown in FIG. 5, an object (100) can be dipped into a bath (102) of liquid phase metallic glass (104) in accordance with embodiments of the invention.

As stated previously, the layer of liquid phase metallic glass can also be spun to facilitate the cooling and/or to eliminate excess material. FIG. 6A demonstrates spinning an object (200) that has been dipped in a bath of liquid phase metallic glass (202) to eliminate excess material and/or to facilitate cooling. Indeed, in many embodiments, the layer of liquid phase metallic glass is spun primarily to get rid of excess liquid, which can inhibit the quality of the surface finish. FIGS. 6B and 6C show exemplary embodiments demonstrating the utility of such spinning techniques. Specifically, FIG. 6B shows an uncoated Ti surface (right), a Ti surface that has been immersion coated without spinning (center) and shows significant surface defects as the result of running and dripping of the material, and a Ti surface that has been immersion coated with spinning (left) and shows substantial improvement. Also included in FIG. 6C is an image of a steel surface that has been immersion coated with spinning to show that the improvement occurs across multiple material systems.

A system for dipping an object in a bath of liquid phase metallic glass in an inert environment to form a layer of devitrified metallic glass in accordance with embodiments of the invention is illustrated in FIG. 7. In particular, the system (300) includes an airlock (302) that initially houses the object(s) to be coated (304). When the object (304) is ready to be coated, it is transferred to the chamber for depositing the metallic glass layer (306). The chamber (306) is substantially an inert environment. A purging line (308) is used to substantially fill the chamber (306) with an inert substance such as argon, helium, neon, and/or nitrogen, and thereby create and preserve the substantially inert environment. The inert environment can prevent the contamination of the coating layer. The chamber 306 further includes a bath of liquid phase metallic glass (310), heating elements (312) to heat the bath of liquid phase metallic glass, and a source for emitting cooling gas (314) to cool an object coated in liquid phase metallic gas. The object (304) is shown having been dipped in the bath of liquid phase metallic glass (310), and ready for cooling by the source for emitting cooling gases (314). Of course, it is not necessary that the entire object be dipped in the bath of liquid phase metallic glass; in many embodiments, at least a portion of the object is dipped in the liquid phase metallic glass. As can be inferred, dipping the object (304) (or at least a portion of it) in the bath of liquid phase metallic glass (310) is sufficient to apply a relatively substantial volume of liquid phase metallic glass to the object, e.g. such that a smooth coating layer can develop.

It should of course be understood that any suitable metallic glass can be used, and that any suitable technique for cooling can be used in accordance with embodiments of the invention. For example, it is not necessary to use a source of cooling gases to cool the layer of metallic glass. The layer of metallic glass can be cooled simply by thermal conduction for instance.

Generally, these dipping techniques can be substantially advantageous in many respects; for example, they can provide for an efficient and economical way of developing a smooth devitrified metallic glass coating.

Although embodiments of techniques for immersion coating by dipping have been described above, liquid phase metallic glass can also be poured over an object to develop a layer of devitrified metallic glass in accordance with embodiments of the invention. A system for pouring liquid phase metallic glass over an object develop a layer of metallic glass is illustrated in FIG. 8. In particular, the system (500) includes a chamber for depositing the metallic glass alloy (502), a source of liquid phase metallic glass (504), a vat for receiving excess poured liquid phase metallic glass alloy (506), a purging line (508) to maintain a substantially inert environment, and a source for cooling the layer of liquid phase metallic glass (510). Accordingly, a layer of devitrified metallic glass can be formed in accordance with embodiments of the invention by pouring the liquid phase metallic glass over an object (512), and cooling the layer of liquid phase metallic glass sufficiently slowly to form a solid phase layer of devitrified metallic glass. Again, it is not necessary that liquid phase metallic glass be poured over the entire object; in many embodiments, liquid phase metallic glass is poured over at least a portion of the object. As before, any suitable metallic glass forming alloy can be used, and any suitable cooling techniques can be used, in accordance with embodiments of the invention. For example, it is not necessary to use a source of cooling gases to control the temperature of the layer of metallic glass. Such pouring techniques can also provide for an efficient and economical way to develop devitrified metallic glass layers. The above-described dipping and pouring techniques can be used in a myriad of applications whereby devitrified metallic glass coating layers are desired; some of these applications are now discussed below.

Applications for Metallic Glass Coatings

The above described techniques can be used to effectively and efficiently implement metallic glass coatings, which can possess favorable materials properties. For example, devitrified metallic glasses can be developed to possess corrosion resistance, wear resistance, and sufficient resistance to brittle failure, and otherwise favorable structural properties. Additionally, as mentioned above, techniques in accordance with embodiments of the instant invention can implement devitrified metallic glass coating layers that have a smooth surface, which can be aesthetically appealing and/or utilitarian. Thus, in many embodiments of the invention, objects are coated with devitrified metallic glass layers to enhance the functionality of the object. For example, in many embodiments, electronic casings are coated with devitrified metallic glass layers using any of the above described techniques.

A system for developing a devitrified metallic glass coating for a phone casing in accordance with embodiments of the invention is illustrated in FIG. 9. In particular, the system (600), and its operation, is similar to that seen in, and described with respect to, FIG. 5, except that a phone case (602) is the object that is coated in a devitrified metallic glass layer. In this way, the coating can conform to the shape of the casing, and accordingly, it can be as if the casing had been fabricated entirely from the devitrified metallic glass. However, the overall cost of production of the casing coated in devitrified metallic glass may be cheaper than if the casing had been entirely fabricated from devitrified metallic glass. Additionally, if the thickness of the devitrified metallic glass coating layer is thinner than the plastic zone size of the devitrified metallic glass, the coating layer can be resistant to cracking. Further, if the base material of the coated object is relatively soft (e.g. if it is made from aluminum), the softness can provide for an enhanced toughness for the coated object as a whole. In this way, the coated object can have better structural properties as compared to if it were made from either the metallic glass or the soft base metal individually. Generally, the devitrified metallic glass coating can provide improved structural characteristics and an improved cosmetic finish.

Of course it should be understood that although the coating of a phone casing has been described above, any suitable object can be coated using the techniques described herein in accordance with embodiments of the invention. For example, devitrified metallic glass coating layers can be deposited on any of the following objects in accordance with embodiments of the invention: laptop case, electronic case, a mirror, sheet metal, metal foams, graphite parts, parts made from refractory metals, aluminum parts, pyrolyzed polymer parts, titanium parts, steel parts, knives, gears, golf clubs, baseball bats, watches, jewelry, miscellaneous metal tools, biomedical implants, etc. Generally, any suitable objects can take advantage of the above-described techniques for developing devitrified metallic glass layers. Note that biomedical are especially well-suited for the techniques described herein as they can take advantage of the hardness and corrosion resistance that devitrified metallic glasses can offer, as well as their resistance to corrosion. Resistance to corrosion is particularly important in biomedical applications because of the potential for corrosion fatigue, which can result from corrosive biological environments. Accordingly, biomedical parts can be fabricated from metal, coated with devitrified metallic glass; in this way, the devitrified metallic glass can provide resistance to corrosion, while the underlying metal can be sufficiently resistant to corrosion fatigue. Additionally, porous foams are also well suited for the dipping techniques described above, which can enable a substantial portion of the exposed surfaces within a porous foam to be sufficiently coated.

Of course it should be understood that the application of relatively substantial volumes of liquid phase metallic glass to an object can be instituted in ways other than those corresponding to the immersion techniques described above in accordance with embodiments of the invention. For instance, spraying techniques can be implemented.

A system for coating the inside of a pipe with a metallic glass layer using a spraying technique in accordance with embodiments of the invention is illustrated in FIG. 10. In particular the system (700) includes a vessel (702) for housing a liquid phase metallic glass, a tubing (704) for transporting the liquid phase metallic glass, and a spray mechanism (706) for spraying liquid phase metallic glass to the inside of a piping (708). The spray mechanism (706) applies relatively substantial volumes of liquid phase metallic glass such that a smooth coating layer can develop. Any suitable techniques for controlling the temperature of the applied liquid phase metallic glass so that it forms a solid phase devitrified metallic glass can be implemented. As mentioned above, coating the inside of a piping with a devitrified metallic glass layer can be beneficial in a number of respects. For example, devitrified metallic glass coatings have advantageous structural characteristics as well as corrosion resistance. Moreover, the smooth coating layer can promote laminar flow while the pipe is in operation.

It should of course be understood that although several techniques have been discussed above with respect to developing devitrified metallic glass coating layers, by applying relatively substantial volumes of liquid phase metallic glass, any number of techniques can be used to do so in accordance with embodiments of the invention. In essence, the above-descriptions are meant to be illustrative and not comprehensive.

Additionally, although much of the above-discussion has been focused on developing devitrified metallic glass coating layers, free-standing devitrified metallic glass layers can also be developed in accordance with embodiments of the invention and this is now discussed. In many embodiments, free standing sheets of devitrified metallic glass layers are fabricated by depositing relatively substantial volumes of liquid phase metallic glass onto a substrate, e.g. such that a smooth coating layer can develop, allowing the liquid phase metallic glass to cool and thereby form a solid phase layer of devitrified metallic glass, and separating the solid phase metallic glass from the substrate layer. A system for fabricating free-standing sheets of devitrified metallic glass is illustrated in FIGS. 11A and 11B. In particular, the system (800) includes a chamber that houses a substantially inert environment, a purging line (804) used to substantially fill the chamber (802) with an inert substance such as argon, helium, and/or neon, and thereby create and preserve the substantially inert environment, a vessel (806) containing liquid phase metallic glass, heating elements (808) to maintain the liquid phase metallic glass, temperature control elements to control the temperature of the poured liquid phase metallic glass, and a substrate (812). In essence, liquid phase metallic glass from the vessel is poured onto the substrate (812), and is then allowed to cool so as to form a layer of solid phase devitrified metallic glass (814). In the illustrated embodiment, it is shown that the substrate is disposed on a conveyer belt that transports the poured liquid phase metallic glass to the temperature control elements. Thereafter, as shown in FIG. 11B, the solid phase devitrified metallic glass layer (814) is removed from the substrate (812). The devitrified metallic glass layer can be removed using any suitable techniques, e.g. cutting. Thus, a free standing layer of metallic glass can be obtained. Of course, as before, any metallic glass can be used, and any temperature control techniques can be used.

In many embodiments of the invention, forming techniques are introduced into processes for fabricating devitrified metallic glass layers. For example, rolling wheels can be used. A rolling wheel used to form a free standing sheet in accordance with embodiments of the invention is illustrated in FIG. 12. The system (900) depicted in FIG. 12 is similar to that seen in FIGS. 11A and 11B except that it further includes a rolling wheel (902). The rolling wheel can be used to further form the devitrified metallic glass layer into a desired shape prior to its solidification. Of course it should be understood that any forming tools can be used in accordance with embodiments of the invention, not just rolling wheels. Additionally, it should be understood that such forming techniques can be used in conjunction with any of the above-described techniques in accordance with embodiments of the invention, not just those with respect to forming free standing layers of devitrified metallic glass.

Selection of Metallic Glass-Based Materials

Although the above description has focused on the methods of immersion coating layers of a solid phase devitrified metallic glass-based material on an object, it should be understood that not all metallic glass-based materials can be used with methods. Indeed, only metallic glass-based materials which can be formed into a fully amorphous ribbon may be used with the current systems and methods, that is, alloys that can be form fully amorphous in at least a 15 micron thick ribbon via melt spinning or splat quenching, or other fast cooling rate laboratory scale process (˜10⁶ K/s).

Suitable metallic glasses include copper-zirconium based metallic glasses, titanium-based metallic glasses, iron-based metallic glasses, nickel-based metallic glasses, and zirconium based metallic glasses. In many embodiments, the metallic glass is one of: Cu₄₀Zr₄₀Al₇Be₁₀Nb₃, Cu₄₅Zr₄₅Al₅Y₂Nb₃, Cu_(42.5)Zr_(42.5)Al7Be5Nb3, Cu_(41.5)Zr_(41.5)Al₇Be₇Nb₃, Cu_(41.5)Zr_(41.5)Al₇Be₇Cr₃, Cu₄₄Zr₄₄Al₅Ni₃Be₄, Cu_(46.5)Zr_(46.5)Al₇, Cu₄₃Zr₄₃Al₇Ag₇, Cu_(41.5)Zr_(41.5)Al₇Be₁₀, Cu₄₄Zr₄₄Al₇Be₅, Cu₄₃Zr₄₃Al₇Be₇, Cu₄₄Zr₄₄Al₇Ni₅, Cu₄₀Zr₄₀Al₁₀Be₁₀, Cu₄₁Zr₄₀Al₇Be₇Co₅, Cu₄₂Zr₄₁Al₇Be₇CO₃, Cu_(47.5)Zr₄₈Al₄Co_(0.5), Cu₄₇Zr₄₆Al₅Y₂, Cu₅₀Zr₅₀, Ti_(33.18)Zr_(30.51)Ni_(5.33)Be_(22.88)Cu_(8.1), Ti₄₀Zr₂₅Be₃₀Cr₅, Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈, Ti₄₅Zr₁₆Ni₉Cu₁₀Be₂₀, Zr_(41.2)Ti_(13.8)Cu_(12.5)Ni₁₀Be_(22.5), Zr_(52.5)Ti₅Cu_(17.9)Ni_(14.6)Al₁₀, Zr_(58.5)Nb_(2.5)Cu_(15.6)Ni_(12.8)Al_(10.3), Zr₅₅Cu₃₀Al₁₀Ni₅, Zr₆₅Cu_(17.5)Al_(7.5)Ni₁₀, ZrAlCo, Zr_(36.6)Ti_(31.4)Nb₇Cu_(5.9)Be_(19.1), Zr₃₅Ti₃₀Cu_(8.25)Be_(26.75), and mixtures thereof. These alloys have demonstrated sufficient glass forming ability. Of course, although several metallic glass alloys are listed, embodiments in accordance with the instant invention are not limited to using these alloys. Indeed, any suitable metallic glass having a critical cooling rate of no greater than 10⁶K/s can be used in accordance with embodiments of the invention.

Importantly, because the method does not rely on high cooling rates such that a solid phase metallic glass coating results, conventional cooling processes may be used. Moreover, it is not necessary to restrict the choice of metallic glass composition to those with relatively low critical cooling rate, e.g., a ‘bulk metallic glass’, where the critical cooling rate of the metallic glass alloy is less than approximately 1000 K/s. Of course although a particular threshold value is referenced, any suitable metallic glass can be implemented in accordance with embodiments of the invention. Additionally, although the critical cooling rate can be used as a measure of glass forming ability in accordance with embodiments of the invention, any suitable measure of glass forming ability can be used. For instance, the thickness of a part that can be readily formed from a metallic glass using standard casting procedures can be used to judge the metallic glass's glass forming ability, as described above. Accordingly, in many embodiments, a metallic glass is used that can readily be cast into parts having a thickness of greater than approximately 15 micron thick.

Metallic glass forming alloys are designed around deep eutectics, also known as very low melting temperature. One exemplary alloy system is the Ti—Be binary. As shown in the phase diagram provided in FIG. 13, this system shows a very deep eutectic temperature at 30 atomic % Be. This is advantageous when creating a coating through immersion because the lower melting temperature prevents melting of the object being coated. Also, unlike other low melting temperature alloys, metallic glasses almost always crystallize into ordered, hard, high-strength phases, which are advantageous for structural coatings. Moreover, these crystalline metallic glass forming alloys are typically brittle and would be hard to machine or apply as a coating without the current technique

In particular, limiting the metallic glass-based materials to those that can be formed into an amorphous part at critical cooling rates <10⁶ K/s, in accordance with embodiments allows for an immersion system and process that can take advantage of several properties of these alloys that would not be possible with conventional alloys or poorer glass forming alloys. For example, as shown in FIG. 14, metallic glass-forming alloys are typically designed around low melting temperatures (e.g., typically less than 800° C.), which means that they can be used as a molten bath or immersion without melting a part that is subjected to the coating process, i.e., they can be applied at relatively low temperatures. Indeed, nearly all BMGs have lower melting temperatures than their constituents, e.g., alloys with melting temperatures <1200° C. What is more, the low melting temperature of these alloys is unaffected by their crystal structure.

In the current patent, coatings are applied to alloys that could be formed amorphous with sufficient cooling rate, but are purposely forced to form crystalline structured to take advantage of the properties of the crystalline state. This process is helped by the fact that metallic glass-forming alloys undergo a slow crystallization process. Specifically, FIG. 15 shows three x-ray diffraction scans from the same alloy cooled at different rates, each showing varying levels of crystallization. This means that the nature of the crystal structure of the coatings formed from these materials can be controlled to form a desired structure (e.g., a nanocrystalline structure, which is known to have very high strength due to the Hall-Petch relation).

Finally, it has been surprisingly discovered that devitrified metallic glass forming alloys are comprised of very hard, ordered phases, which improve wear resistance and hardness compared to amorphous coatings. This is confirmed by the data table presented in FIG. 16, which provides a comparison of a family of metallic glass-based materials in amorphous (A), crystalline (X) and composite (C) states. As shown, crystallization does not reduce hardness and in some cases actually improves it. Accordingly, the de-vitrified coating will have very high hardness and high stiffness, harder and stiffer than many metallic glasses, which is advantageous for many applications.

These observations were further confirmed by a direct study of a Ti-crystalline BMG coating. As shown in FIG. 17, which summarizes the results from the study, the application of a devitrified Ti-based BMG coating increases the hardness of Ti-6-4 by 25%. Moreover, the hardness of the coating is almost the same as the hardness of the same injection molded BMG. Accordingly, contrary to convention wisdom a coating of the devitrified BMG may be used as a coating in certain applications, and these crystalline coatings can be used to make a softer metal having the hardness of BMG (or harder), thus improving wear resistance and allowing for higher temperatures to be used.

Note that this technique can further take advantage of the fact that certain metallic glass alloys, especially bulk metallic glasses, have excellent wetting characteristics. For example, many bulk metallic glasses have excellent wetting characteristics with respect to aluminum, titanium, steel, cobalt, graphite, quartz and silicon-carbide. Accordingly, in many embodiments of the invention, the object that is the subject of the application of the liquid phase metallic glass includes one of: aluminum, titanium, steel, cobalt, graphite, quartz, silicon-carbide, and mixtures thereof. Because the viscosity of the alloy in the liquid state will be high even if the coating is de-vitrified, the thickness and wetting angle of the coating will be similar to a fully amorphous layer. Accordingly, the appearance of a de-vitrified will be similar to an amorphous coating in thin layers so the part will look like it is made from metallic glass but does not require the high cooling rate.

The above description is meant to be illustrative and not meant to be a comprehensive definition of the scope of invention. In general, as can be inferred from the above discussion, the above-mentioned concepts can be implemented in a variety of arrangements in accordance with embodiments of the invention. Accordingly, although the present invention has been described in certain specific aspects, many additional modifications and variations would be apparent to those skilled in the art. It is therefore to be understood that the present invention may be practiced otherwise than specifically described. Thus, embodiments of the present invention should be considered in all respects as illustrative and not restrictive. 

What claimed is:
 1. A method of fabricating a layer of devitrified metallic glass comprising: providing a liquid phase metallic glass having a critical cooling rate of less than 10⁶ K/s; applying a liquid phase metallic glass to an object, wherein applying the coating layer comprises immersing at least a portion of the object such that the object is wetted by the liquid phase metallic glass to form a layer of liquid phase metallic glass on the outer surface thereof; and solidifying the layer of liquid phase metallic glass-forming alloy such that a solid phase devitrified metallic glass-forming coating is formed therefrom.
 2. The method of claim 1 where the grain size of the coating is nanocrystalline with an average grain size from 10 nanometers to 1000 nanometers.
 3. The method of claim 2 where the grain size is greater than 1 micrometer.
 4. The method of claim 1 where the coating crystallizes during cooling from the liquid phase.
 5. The method of claim 1 further comprising applying an external heat source to heat the object during solidifying.
 6. The method of claim 1 further comprising: quenching the liquid phase at a cooling rate faster than the critical cooling rate of the liquid phase metallic glass to form a solid phase metallic glass coating; heating the solid phase metallic glass coating to a processing temperature above the glass transition temperature of the metallic glass and holding the metallic glass coating at the processing temperature to form a devitrified metallic glass-forming coating; and cooling the devitrified metallic glass-forming coating to below the glass transition temperature.
 7. The method of claim 1 where the devitrified coating has a hardness that is at least 10% higher than the amorphous phase of the same alloy.
 8. The method of claim 1 where the devitrified coating has a Young's modulus that is at least 10% higher than the amorphous phase of the same alloy.
 9. The method of claim 1 where the metallic glass-forming alloy is applied to an object that is at a higher temperature than the liquidus temperature of the metallic glass-form ing alloy causing it to melt and wet the object.
 10. The method of claim 1 where the devitrified coating has a lower surface roughness than the object to which the liquid phase metallic glass is applied.
 11. The method of claim 1 where the immersion of the object comprises one of the methods selected from the group consisting of dipping, pouring and spraying.
 12. The method of claim 1 where the object being coated is made from metal, polymer, ceramic, glass, or mixtures thereof.
 13. The method of claim 1, wherein the thickness of the coating layer is greater than 50 micrometers.
 14. The method of claim 1, wherein the thickness of the coating layer is greater than 1 mm.
 15. The method of claim 1 where the coating process is done under a vacuum or inert environment.
 16. The method of claim 1 where the coating does not exhibit a glass transition temperature when heated.
 17. The method of claim 1 further comprising spinning the object during the applying and solidifying.
 18. The method of claim 1, wherein the object comprises one of aluminum, titanium, steel, cobalt, graphite, quartz, silicon carbide, and mixtures thereof.
 19. The method of claim 1, wherein the metallic glass has a melting temperature of less than 800° C.
 20. A method of fabricating a layer of devitrified metallic glass comprising: providing a liquid phase metallic glass having a critical cooling rate of less than 10⁶ K/s and a melting temperature of less than 800° C.; applying a liquid phase metallic glass to an object, wherein applying the coating layer comprises immersing at least a portion of the object such that the object is wetted by the liquid phase metallic glass to form a layer of liquid phase metallic glass on the outer surface thereof; and solidifying the layer of liquid phase metallic glass-forming alloy such that a solid phase devitrified metallic glass-forming coating is formed therefrom. 